Stripe’s Developer Productivity team rolled out rubyfmt, a Rust-based zero-config Ruby autoformatter, across a 25-million-line monorepo in a single overnight run.
Key Takeaways
rubyfmt is Rust-based and zero-config, designed for speed at the scale of the world’s largest Ruby codebase.
The team chose a Saturday cutover to minimize merge conflicts rather than an incremental rollout.
Their approach prioritized the hardest syntax cases first, reasoning that solving edge cases covers the long tail.
High test suite confidence was required before committing a diff too large for GitHub to render.
Hacker News Comment Review
Commenters split on the all-at-once vs. incremental strategy: incremental rollout via open-PR exclusion scripts can reformat ~95% of files safely, while a single-day cutover enables clean blame.ignoreRevsFile usage but forces conflict resolution.
The “overnight” framing drew skepticism: one commenter benchmarked clang-format on 21M lines of Chromium in under 6 minutes on a 2014 Xeon, suggesting the timeline reflects process overhead, not raw formatter speed.
A core formatter reliability tip surfaced: the Dart formatter walks unformatted and formatted output in parallel, comparing non-whitespace characters to catch any semantic corruption – commenters suggested AST diffing as a stronger check.
Notable Comments
@hobofan: describes an incremental PR-exclusion script strategy that safely reformatted 95% of files before the final sweep.
@munificent: explains Dart formatter’s whitespace-skipping sanity check as a model for catching silent reformatter bugs.
